This invention relates to the fabrication of integrated circuits on silicon, and specifically to the provision of a low temperature, high quality silicon dioxide layer formed from the oxidation of silicon.
Conventional techniques for oxidation of silicon require high temperatures, e.g., greater than 800xc2x0 C., for long periods of time in an oxidizing ambient such as O2, N2O, or NO. During such oxidation, diffusion of elements occurs within the substrate, and fabrication sequences must be tailored to accommodate such diffusion. The ability to perform an oxidation at much lower temperatures, without sacrificing the quality, is a tremendous benefit to the semiconductor industry.
Thermal oxidations have different oxidation rates, depending on silicon crystal orientation. A 30 nm silicon oxide layer is grown in one hour at 900xc2x0 C. in a dry O2 ambient on silicon (111) whereas it only grows to about 21 nm on silicon (100) in the same amount of time at the same temperature. In a wet oxidation at 900xc2x0 C. for 1 hour, the thickness is approximately 215 nm on silicon (111), but only 150 nm on silicon (100). These differences are critical for an oxidation for shallow trench isolation.
High temperature oxidations are also know to cause stacking faults at the silicon interface and extensive annealing steps are required to minimize the impact of such faults on device performance. This adds to the time and cost of processing for gate oxide formation.
An acceptable method of oxidizing silicon at low temperatures for manufacturing purposes currently does not exist. There are known methods of oxidizing silicon at low temperatures, such as plasma oxidation or oxidation with a radial slot-line antennae, however, these methods produce large quantities of ions as well as radicals which can damage the silicon surface and degrade the oxide quality. Oxidation with ozone, or ozone with ultraviolet light, has been reported, but the resultant films were observed to be self-limiting in thickness.
Ozone may be photodissociated with ultra-violet light to generate oxygen radicals, however, the atmospheric pressure used in the system allows oxygen radicals to collisionally recombine to ozone. The results are thus severely handicapped by the lack of the reactive oxygen radicals. Nevertheless, enhanced oxidation rates and good stoichiometric oxide are achieved which are superior to the state-of-the-art processes. In the above-identified related patent application, Ser. No. 10/164,924, O2, N2O or NO are photodissociated by a xenon excimer laser to produce the oxygen radical O(1D), or Oxe2x88x92 ions.
Ishikawa et al., Low Temperature Thermal Oxidation of Silicon in N2O by UV irradiation, Jpn. J. of Appl. Phys., 28, L1453 (1989), disclose photodissociation of N2O with ultra-violet light at temperatures between 200xc2x0 C. and 500xc2x0 C.
In Nayer et al., Atmospheric Pressure, Low Temperature ( less than 500xc2x0 C.) UV/Ozone Oxidation of Silicon, Electronics Letters, 26, 205 (1990), an ultra thin oxide layer, approximately 40 xc3x85 thick, is formed on a silicon substrate by photodissociation of O3 by ultra-violet light at temperatures below 500xc2x0 C.
Hirayama et al., Low Temperature Growth of High-Integrity Silicon Oxide Films by Oxygen Radical Generated in High-Density Krypton Plasma, IEDM Tech. Dig. p249, (1999), describes oxides grown on silicon at approximately 400xc2x0 C. by low ion bombardment and high plasma density.
Saito et al., Advantage of Radical Oxidation for Improving Reliability of Ultra-Thin Gate Oxide, 2000 Symposium on VLSI Technology, T18-2, (2000) describes formation of oxide with plasma oxidation with a radial slot line antennae.
A method of low-temperature oxidation of a silicon substrate includes placing a silicon wafer in a vacuum chamber; maintaining the silicon wafer at a temperature of between about room temperature and 350xc2x0 C.; introducing an oxidation gas in the vacuum chamber including introducing an oxidation gas taken from the group of oxidation gases consisting of N2O, NO, O2 and O3; dissociating the oxidation gas into radical oxygen with a xenon laser generating light at a wavelength of about 172 nm with a power of between about 3 mW/cm2 to 20 mW/cm2, and flowing the radical oxygen over the silicon wafer; and forming an oxide layer on at least a portion of the silicon wafer. The wafer may be further annealed at a temperature of between about 600xc2x0 C. to 750xc2x0 C. for between about one to ten minutes.
It is an object of the invention to form an oxide layer on a silicon substrate at a temperature below 600xc2x0 C.
Another object of the invention is to provide free oxygen radicals by photodissociation of O2 or O3 with a xenon excimer laser.
A further object of the invention is to provide a method of increasing oxide growth by the application of a small negative voltage to the silicon wafer.
This summary and objectives of the invention are provided to enable quick comprehension of the nature of the invention. A more thorough understanding of the invention may be obtained by reference to the following detailed description of the preferred embodiment of the invention in connection with the drawings.